VIEW DEFINITION STATEMENTS¶
CREATE VIEW¶
A view is a virtual table that does not exist physically. You can create a view by using an existing table or a query. VIEW and VCLASS are used interchangeably.
Use CREATE VIEW statement to create a view. Regarding writing view name, see Identifier.
CREATE [OR REPLACE] {VIEW | VCLASS} view_name
[<subclass_definition>]
[(view_column_name [COMMENT 'column_comment_string'], ...)]
[INHERIT <resolution>, ...]
[AS <select_statement>]
[WITH CHECK OPTION]
[COMMENT [=] 'view_comment_string'];
<subclass_definition> ::= {UNDER | AS SUBCLASS OF} table_name, ...
<resolution> ::= [CLASS | TABLE] {column_name} OF superclass_name [AS alias]
OR REPLACE: If the keyword OR REPLACE is specified after CREATE, the existing view is replaced by a new one without displaying any error message, even when the view_name overlaps with the existing view name.
view_name: specifies the name of a view to be created. It must be unique in a database.
view_column_name: defines the column of a view.
AS <select_statement>: A valid SELECT statement must be specified. A view is created based on this.
WITH CHECK OPTION: If this option is specified, the update or insert operation is possible only when the condition specified in the WHERE clause of the <select_statement> is satisfied. Therefore, this option is used to disallow the update of a virtual table that violates the condition.
view_comment_string: specifies a view’s comment.
column_comment_string: specifies a column’s comment.
CREATE TABLE a_tbl (
id INT NOT NULL,
phone VARCHAR(10)
);
INSERT INTO a_tbl VALUES(1,'111-1111'), (2,'222-2222'), (3, '333-3333'), (4, NULL), (5, NULL);
--creating a new view based on AS select_statement from a_tbl
CREATE VIEW b_view AS SELECT * FROM a_tbl WHERE phone IS NOT NULL WITH CHECK OPTION;
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
--WITH CHECK OPTION doesn't allow updating column value which violates WHERE clause
UPDATE b_view SET phone=NULL;
ERROR: Check option exception on view b_view.
The below updates the old view’s definition, In addition, this adds a view’s comment.
--creating view which name is as same as existing view name
CREATE OR REPLACE VIEW b_view AS SELECT * FROM a_tbl ORDER BY id DESC COMMENT 'changed view';
--the existing view has been replaced as a new view by OR REPLACE keyword
SELECT * FROM b_view;
id phone
===================================
5 NULL
4 NULL
3 '333-3333'
2 '222-2222'
1 '111-1111'
The below adds a comment to a view’s columns.
CREATE OR REPLACE VIEW b_view(a COMMENT 'column id', b COMMENT 'column phone') AS SELECT * FROM a_tbl ORDER BY id DESC;
Condition for Creating Updatable VIEW¶
A virtual table is updatable if it satisfies the following conditions:
The FROM clause must include the updatable table or view only.
In versions lower than CUBRID 9.0, only one updatable table can be included to the FROM clause it requires. However, two tables in parentheses like FROM (class_x, class_y) can be updated since the two were expressed as one table. In version of CUBRID 9.0 or higher, more than one updatable table is allowed. The FROM clause must include only one table or updatable view. However, two tables included in parentheses as in FROM (class_x, class_y) can be updated because they represent one table.
A JOIN syntax can be included.
Note
In versions lower than CUBRID 10.0, you cannot update a view which is created with a JOIN syntax.
The DISTINCT or UNIQUE statement should not be included.
The GROUP BY… HAVING statement should not be included.
Aggregate functions such as SUM or AVG should not be included.
The entire query must consist of queries that can be updated by UNION ALL, not by UNION. However, the table should exist only in one of the queries that constitute UNION ALL.
If a record is inserted into a view created by using the UNION ALL statement, the system determines into which table the record will be inserted. This cannot be done by the user. To control this, the user must manually insert the row or create a separate view for insertion.
Even when all rules above are satisfied, columns that contains following contents cannot be updated.
Path expressions (example: tbl_name.col_name)
Numeric type column that includes an arithmetic operator
Even though the column defined in the view is updatable, a view can be updated only when an appropriate update authorization is granted on the table included in the FROM clause. Also there must be an access authorization to a view. The way to grant an access authorization to a view is the same to grant an access authorization to a table. For details on granting authorization, see GRANT.
View’s COMMENT¶
You can specify a view’s comment as follows.
CREATE OR REPLACE VIEW b_view AS SELECT * FROM a_tbl ORDER BY id DESC COMMENT 'changed view';
You can see the specified comment of a view by running this syntax.
SHOW CREATE VIEW view_name;
SELECT vclass_name, comment from db_vclass;
Or you can see the view’s comment with ;sc command which displays the schema in the CSQL interpreter.
$ csql -u dba demodb
csql> ;sc b_view
Also, you can add a comment for each column of the view.
CREATE OR REPLACE VIEW b_view (a COMMENT 'a comment', b COMMENT 'b comment')
AS SELECT * FROM a_tbl ORDER BY id DESC COMMENT 'view comment';
To see how to change a comment of a view, refer to ALTER VIEW syntax on the below.
ALTER VIEW¶
ADD QUERY Clause¶
You can add a new query to a query specification by using the ADD QUERY clause of the ALTER VIEW statement. 1 is assigned to the query defined when a virtual table was created, and 2 is assigned to the query added by the ADD QUERY clause.
ALTER [VIEW | VCLASS] view_name
ADD QUERY <select_statement>
[INHERIT <resolution> , ...] ;
<resolution> ::= {column_name} OF superclass_name [AS alias]
view_name: specifies the name of a view where the query to be added.
<select_statement>: specifies the query to be added.
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
4 NULL
5 NULL
ALTER VIEW b_view ADD QUERY SELECT * FROM a_tbl WHERE id IN (1,2);
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
4 NULL
5 NULL
1 '111-1111'
2 '222-2222'
AS SELECT Clause¶
You can change the SELECT query defined in the virtual table by using the AS SELECT clause in the ALTER VIEW statement. This function is working like the CREATE OR REPLACE statement. You can also change the query by specifying the query number 1 in the CHANGE QUERY clause of the ALTER VIEW statement.
ALTER [VIEW | VCLASS] view_name AS <select_statement> ;
view_name: specifies the name of a view to be modified.
<select_statement>: specifies the new query statement to replace the SELECT statement defined when a view is created.
ALTER VIEW b_view AS SELECT * FROM a_tbl WHERE phone IS NOT NULL;
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
CHANGE QUERY Clause¶
You can change the query defined in the query specification by using the CHANGE QUERY clause reserved word of the ALTER VIEW statement.
ALTER [VIEW | VCLASS] view_name
CHANGE QUERY [integer] <select_statement> ;
view_name: specifies the name of a view to be modified.
integer: specifies the number value of the query to be modified. The default value is 1.
<select_statement>: specifies the new query statement to replace the query whose query number is integer.
--adding select_statement which query number is 2 and 3 for each
ALTER VIEW b_view ADD QUERY SELECT * FROM a_tbl WHERE id IN (1,2);
ALTER VIEW b_view ADD QUERY SELECT * FROM a_tbl WHERE id = 3;
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
4 NULL
5 NULL
1 '111-1111'
2 '222-2222'
3 '333-3333'
--altering view changing query number 2
ALTER VIEW b_view CHANGE QUERY 2 SELECT * FROM a_tbl WHERE phone IS NULL;
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
4 NULL
5 NULL
4 NULL
5 NULL
3 '333-3333'
DROP QUERY Clause¶
You can drop a query defined in the query specification by using the DROP QUERY of the ALTER VIEW statement.
ALTER VIEW b_view DROP QUERY 2,3;
SELECT * FROM b_view;
id phone
===================================
1 '111-1111'
2 '222-2222'
3 '333-3333'
4 NULL
5 NULL
DROP VIEW¶
You can drop a view by using the DROP VIEW clause. The way to drop a view is the same as to drop a regular table. If you also specify IF EXISTS clause, no error will be happened even if a target view does not exist.
DROP [VIEW | VCLASS] [IF EXISTS] view_name [{ ,view_name , ... }] ;
view_name: specifies the name of a view to be dropped.
DROP VIEW b_view;
RENAME VIEW¶
You can change the view name by using the RENAME VIEW statement.
RENAME [VIEW | VCLASS] old_view_name {AS | TO} new_view_name[, old_view_name {AS | TO} new_view_name, ...] ;
old_view_name: specifies the name of a view to be modified.
new_view_name: specifies the new name of a view.
The following example shows how to rename a view name to game_2004.
RENAME VIEW game_2004 AS info_2004;
COMMENT Clause¶
You can change a view’s comment, columns’ comment, or attributes’ comment with COMMENT clause of ALTER VIEW syntax.
view_name: Specifies the name of a view to be modified.
column_name: Specifies the name of a column to be modified.
view_comment_string: Specifies a view’s comment.
column_comment_string: Specifies a column’s comment.
The following example shows how to change a view’s comments.
You can change the column comment by specifying one or more columns after the ON COLUMN keyword. The following example shows how to change a column’s comments.
Below is a syntax to show a column’s comment. But the SHOW CREATE VIEW statement shows only view comments.
You can see this comment with the “;sc view_name” command in the CSQL interpreter.